An X-ray CT apparatus, which provides information on a subject as images based on intensity of X-rays transmitted through the subject, plays an important role in many medical actions including diagnosis and treatment of diseases and surgical planning.
Recent X-ray CT apparatuses use a technique known as dual energy scanning. The dual energy scanning as referred to herein is a technique for acquiring images by scanning a subject using two different types of X-ray tube voltage. CT which uses dual energy scanning is referred to as “dual energy CT.”
The X-ray CT apparatus which uses the dual energy scanning technique separates materials based on information obtained using two different types of X-ray tube voltage and can thereby obtain various images such as monochromatic X-ray images, density images, effective atomic number images, or artifact-free images (images with reduced artifacts). Note that the X-rays used in this case are continuous spectrum X-rays containing various energies and having a specific energy distribution.
In conventional art, various types of images are obtained using the dual energy scanning technique. However, there is a problem in that it is difficult to select one of conceivable candidate energies because plural materials are considered to be usually contained in radiographic coverage and optimum energy for diagnostic imaging varies depending on a subject's diagnosis region, materials, and the like. In determining whether or not an energy value is optimum for diagnostic imaging, factors taken into consideration include, for example, whether there are large differences in CT value, providing clear contrast or whether bones and artifacts can be removed.
Suppose, for example, an X-ray CT apparatus generates monochromatic X-ray images using the dual energy scanning technique. When soft tissue contained in radiographic coverage is diagnostically imaged, differences in CT value are large at relatively low energy, providing clear contrast and making it easy to diagnostically image the soft tissue, but the differences in CT value are small at relatively high energy, providing low contrast and making it difficult to diagnostically image the soft tissue. Thus, from the perspective of diagnostic imaging of soft tissue, relatively low energy has to be selected as optimum energy for generation of monochromatic X-ray images. On the other hand, high energy is advantageous to removal of bones and artifacts contained in radiographic coverage. Then, from the perspective of capability to remove bones and artifacts, relatively high energy has to be selected as optimum energy for generation of monochromatic X-ray images. In such cases, it is difficult to select one of conceivable candidate energies.
As another example, suppose metal artifacts (artifacts stemming from man-made objects containing metal) are produced during diagnostic imaging of soft tissue. As described above, from the perspective of diagnostic imaging of soft tissue, relatively low energy has to be selected as optimum energy for generation of monochromatic X-ray images. However, because metal artifacts are reduced at relatively high energies, from the perspective of reducing metal artifacts, relatively high energy has to be selected as optimum energy for generation of monochromatic X-ray images. This also makes it difficult to select optimum energy for generation of monochromatic X-ray images.